The present invention generally relates to the collection and removal of trash or floating debris from waterways and, more particularly, to systems designed for use in combined sewer systems or storm drain conduits to trap water borne trash for removal.
Trash and debris floating on the surfaces of waterways or along shorelines and beaches is a highly visible form of water pollution, which is receiving attention for its adverse, polluting effect and for its unaesthetic appearance on the surfaces of lakes and other water bodies. One type of system for the collecting and removing of floating debris has consisted of arrays of disposable mesh nets installed in receiving bodies of water in the flow path of a sewer outlet, particularly in applications referred to as xe2x80x9cCombined Sewer Overflowsxe2x80x9d or xe2x80x9cCSOsxe2x80x9d. Such systems are described in Vol. 2, No. 3, of Fresh Creek Technologies, Inc. xe2x80x9cShorelinesxe2x80x9d newsletter. Systems of this type are effective in collecting floatables or trash for removal and are shown in Fresh Creek Technologies, Inc. Netting Trashtrap(trademark) Product Bulletin. Improvements in such devices are described in U.S. Pat. No. 5,562,819, owned by the assignee of the present application, which provides an underground, in-line apparatus for trapping and collecting debris in a sewer or storm flow conduit, a secondary trap which provides continued protection when primary collection traps are full, a system which signals when primary bags or nets are full and servicing is required, and a trapping facility in which bags or nets may be replaced without loss of trapping protection during servicing.
More specifically, the device in the patent referred to above includes an enclosure or chamber with an inlet and an outlet each adapted to be connected to a sewer, storm drain conduit or outflow. A debris removing system is disposed within the chamber between the inlet and the outlet for trapping and collecting water borne debris entering at the inlet and thereby providing for an outflow of substantially debris-free water. The enclosure includes an access opening comprising upper doors or hatches or access hatches in the enclosure sized to allow the debris removing system to be removed and replaced. The debris removing system specifically includes a perforated container having an open end facing the inlet of the chamber. The perforated container includes a netting assembly that traps and collects the trash or floating debris. The container is in the form of a netting assembly having a flexible bag-shaped mesh net attached to a frame. The netting assembly is attached to lifting structure having supports or handles for allowing the frame and net to be lifted out when the net is full of captured debris. In some applications, a bypass weir or screen is provided to normally direct flow from the chamber inlet through the open end of the net while allowing flow to bypass the net and flow to the chamber outlet when the net is full of debris.
Sensing and signaling elements are typically provided for sensing and signaling the passage of solid debris around the net when the net is full of debris and is in need of servicing. The sensing and signaling elements may include mechanical structure which permits passage of water, but is displaced by impingement of solid debris flowing around the nets. Displacement of such mechanical structure signals when the net is full of debris, for example, by actuating a visible flag above ground or by actuating an electrical switch which activates an aboveground indicator or remote indicator. The sensing and signaling may include an optical sensor for detecting the passage of debris around the netting assembly. Upon detection of debris, the optical sensor emits a signal indicating that the trap is full of debris. The signal may also activate an aboveground indicator or a remote indicator.
Multiple trap systems are employed in which the enclosure includes side-by-side trap assemblies. Such systems may be configured such that, upon filling of the first trap, the flow and debris can be diverted over a bypass weir disposed between the inlet ends of the first and second traps so that flow is thereby directed through the second trap and overflow debris is trapped and collected. Closure panels may be provided in a stationary frame structure disposed adjacent the inlet ends of the traps in either the single-trap systems or the multitrap systems to restrain debris from flowing through the chamber during servicing.
The reliability of debris removing systems depends on the strength of the mesh nets and on the manner in which the net material is fabricated into the disposable net assemblies. The resultant hydraulic forces are a function of the velocity of the flow of water through the mesh of the nets as well as on the pressure exerted on the debris trapped by the nets. There are many outfalls where extreme forces exist that are too high for standard and commonly available materials or for materials made by normal fabrication practices to last.
Furthermore, the operation of such debris removal systems results in the nets filling with floatable materials over time as one or more overflows occur. In the process, large objects such as plastic bottles and sheets of plastic wrapping materials tend to cover and blind openings of the mesh, which reduces the overall effective area of the filter and results in higher hydraulic loading on the mesh, contributing to a higher pressure drop through the system and increased loads, and excessive forces on the nets.
Accordingly, a need exists for stronger and more reliable mesh nets in the traps of floatable debris collecting systems, and particularly for net assemblies that can be easily constructed and easily replaced.
A primary objective of the present invention is to provide a stronger and more reliable mesh net for the traps of systems for collecting floatable debris than have been provided by the prior art. A further objective of the invention is to provide a reliable net assembly for such systems that can be easily constructed and easily replaced.
According to principles of the present invention, disposable mesh nets are provided for debris traps that can withstand higher level of forces than can nets of the prior art. Such nets are, according to a preferred embodiment, made with a high strength and high stretch yarn and may be provided with reinforcing tape on seams and high stress areas of the net material. The flexible, stretchable mesh material allows for an increase in the free area of the mesh as the nets expand under hydraulic loads as the nets fill. High elasticity materials are those that are elastic enough, either due to their composition or the ways in which they are knitted, to allow the nets to deform when clogged with debris and thereby expand to allow flow paths around the trapped debris to minimize pressure. Nylon that has these properties would, for example, be suitable. The knit of the mesh material yarn is selected to produce the desired aperture size and maximize the breaking strength of the finished material and ability to maintain constant aperture. The material used in the manufacturing process enables the flexible mesh to maintain a consistent percentage of free area as the nets fill and expand. The material is fabricated into the form of a bag-shaped mesh net from flat material with seams that are rolled and stitched to give a strength greater than the knitted material itself.
Further according to principles of the invention, a netting assembly is provided with structure for holding the mouth of the bag-shaped net in an open position and which can be easily and securely attached to the netting material. In the preferred embodiment, the structure includes a one-piece frame that is provided with a strap configured to hold the netting material in place on the frame. The strap fits in a recessed groove molded into the outer perimeter of a generally rectangular molded plastic frame. Rows of raised buttons integrally molded into the frame extend from the bottom of the groove such that the mesh net will be sandwiched between the strap and the buttons. The frame is sized to provide sufficient strength to counter the hydraulic forces on the net. This particular embodiment of the invention is particularly suited to resist hydraulic forces in the dirty environment wherein the netting assemblies trap floating debris from waterways, sewers or storm drain conduits, as the frame assembly requires no removable locks, pins, clamps, brackets or other devices to hold down the netting material to the frame. The structure has a minimum of parts to collect debris while permitting the netting assembly to be loosened from the system with a pair of gloved hands.
In other embodiments, the netting assemblies are provided with a two part molded plastic rectangular frame, the parts of which clamp together with the knitted mesh material around the mouth of the net clamped therebetween, thereby evenly distributing the forces around the mouth of the net and holding the mouth in an open condition. The two part frame uses hole and post members on the respective parts that snap together for easy assembly. In another alternative embodiment, a one part rectangular frame is provided to which four plates having post members clamp into hole members on the frame. These embodiments have limited projections, thereby avoiding the collection thereon of debris with structure that can easily be loosened by gloved hands.
In accordance with certain principles of the invention, the traps are provided with net assemblies having a two-stage filter mesh. The nets for such traps are constructed of an inner net and an outer net. The inner net provides a first layer of mesh having larger aperture mesh openings so that the inner net captures only the larger items of debris, allowing the smaller items to pass through to the outer net or second layer of mesh. The outer net has smaller openings that trap smaller items of debris that pass through the openings of the inner net. The openings in the inner net may, for example, be at least two or three times the dimension of the openings in the outer net, or have an area from about four to ten times the area of the openings in the outer net. The outer net may also have a greater volume than the inner net, for example, at least about one fourth larger than that of the inner net. The two stage filter produces a larger effective filtering capacity, in that the trap does not blind as quickly, holds more material and distributes the hydraulic loads between the two layers resulting in greater overall strength. Further, were the first or inner net to fail, the second or outer net retains the ability to trap additional debris.
These and other objectives and advantages of the present invention will be more readily apparent from the following detailed description of the preferred embodiments of the invention, in which: